1. Field of the Invention
This invention relates to a method for producing concrete, and more specifically, this invention relates to a method for producing quick-setting concrete while simultaneously minimizing the release of carbon dioxide to the atmosphere, said release of carbon dioxide inherent in cement production.
2. Background of the Invention
Concrete is the most widely used material in the construction industry, with road construction being one of its biggest applications. An inherent problem with cement production, however, is the high level of carbon dioxide that is released therewith, per equation 1, infra: EQU 5CaCO.sub.3 +2SiO.sub.2 .fwdarw.(2CaO.2SiO.sub.2)+(3CaO.2SiO.sub.2)+5CO.sub.2 Eq. 1
wherein CaCO.sub.3 is the calcareous material component (e.g., limestone) that is the source of CO.sub.2 out-gassing during calcination, SiO.sub.2 is argillaceous material (e.g., clay, shale, Al.sub.2 O.sub.3.SiO.sub.2), and the products, .beta.-dicalcium silicate (2CaO.SiO.sub.2) and tricalcium silicate (3CaO.SiO.sub.2), are the major binding phases of cement.
The other source of CO.sub.2 during cement production is the combustion of fuel (i.e. coal) that is required to produce the heat for calcining and clinkering the cement components. Generally, the decomposition of natural lime stone into calcium oxide requires reaction temperatures of approximately 850.degree. C.
For each ton of cement produced, approximately one ton of CO.sub.2 is produced by these two sources combined. By 2015, approximately 3,500 million tons of CO.sub.2 will be produced annually worldwide from cement production. In anticipation of this increase, attempts are being made to reverse this trend. The Clean Air Act of 1990, for example, requires that emission of greenhouse gases be reduced to 1990 levels by the turn of the century. To comply with this and other mandates, the cement industry will require new CO.sub.2 emission reducing technologies.
Another drawback of current concrete fabrication processes is the relatively long cure time of concrete. This is a particular problem in the manufacture of precast components. The economics of concrete use would be considerably enhanced if its curing time could be reduced. Current ASTM standards reflect the long setting rates, by requiring that concrete withstand a load factor of 4000 pounds per square inch (psi) after a 28 day set time.
A method of decreasing both CO.sub.2 liberation and curing times previously has been reported in Nature Physical Science Vol. 240, pp. 16-18 (Nov. 6, 1972). The mechanism appears to involve a carbonation reaction in the aqueous film surrounding the surface particles of a concrete structure. During carbonation, impregnated CO.sub.2 reacts with the silicates on the surface of the pores and grains of the structure.
That process exposes curing concrete to predetermined partial pressures of carbon dioxide. The drawbacks to this technique include incomplete carbonation, due to the inner regions of the grains remaining unreacted. Also, from a practical standpoint, it is not convenient to enclose cement structures, such as bridges and roads, in a carbonation chamber during curing.
Previous efforts at direct impregnation using CO.sub.2 has resulted in the formation of a slurry having low pH. This low pH is contrary to the high pH values needed to facilitate the formation of a corrosion protective layer on steel that is used in the production of reinforced concrete structures.
A need exists in the art to ultimately minimize CO.sub.2 burden in the atmosphere during cement production processes and also to minimize the long curing times now associated with concrete structure fabrication processes. An improved process necessarily must include a method for impregnating the entire microstructure of cement with CO.sub.2, and without the need for placing the entire process in a carbonation chamber.